CDMA mobile communication system and communication method

ABSTRACT

A radio communication system having a base station and a plurality of radio terminals, wherein each radio terminal having a transmission request transmits a reservation packet at arbitrary timing through a reservation channel in accordance with a CDMA scheme, and the base station assigns a traffic channel and a time slot to be used to each radio terminal requesting a reservation through a reply packet outputted onto a reply channel. On the reservation channel, a short spreading code corresponding to a matched filter is applied.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a mobile communication systemand a communication method, and more particularly, to a reservationbased mobile communication system, mobile terminal equipment, andcommunication method to which code division multiple access (CDMA) isapplied.

[0003] 2. Description of the Related Art

[0004] Conventionally, a mobile communication system which employs areservation based access control in a frequency division multiple access(FDMA) scheme is known, for example, as described in IEEE Transactionson Communications, Packet Switching in Radio Channels: “Part3-Pollingand (Dynamic) Split-Channel Reservation Multiple Access”, COM-24, 8,(1976), pp. 832-845 (hereinafter called “prior art publication 1”).

[0005] In the reservation based access control, each of mobile terminalshaving a request for data transmission reserves a traffic channel to abase station through a reservation packet. The base station, afterscheduling traffic channels and transmission timing (time slots) to beassigned to these mobile terminals, notifies each of the mobileterminals of transmission timing to be used on an assigned trafficchannel through a reply packet. According to this reservation basedaccess control, collision of packets on the traffic channel can bebasically avoided.

[0006] As another example of reservation based control typecommunication system, for example, JP-A-6-311160, corresponding to U.S.patent application Ser. No. 08/230773 (hereinafter called “prior artpublication 2”) has proposed such a communication system based on a timedivision multiple access scheme.

[0007] However, in the mobile communication systems in which thereservation based access control is applied to FDMA and TDMA schemes, asproposed by prior art publications 1 and 2, since respective mobileterminals send reservation packets through a reservation channelasynchronously with each other, a plurality of reservation packets cancollide with a high possibility. Thus, repetitive retransmission ofreservation packets obliged by the collision of packets constitutes amain cause of degrading the throughput of the entire communicationsystem.

[0008] Meanwhile, as a standard for FPLMTS (Future Public Land MobileTelecommunication Systems), the adoption of the code division multipleaccess scheme is regarded as promising. A CDMA mobile communicationsystem has been proposed, for example, in JP-A-7-38496 corresponding toU.S. patent application Ser. No. 08/375679 (hereinafter called “priorart publication 3”). However, prior art publication 3 does not provideany useful information for solving the problem of a degraded throughputin the reservation based access control.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a mobilecommunication system and a communication method which employ areservation based access control to realize a high throughput.

[0010] It is another object of the present invention to provide CDMAmobile terminal equipment and base station which solve the problem ofcollision of reservation packets to realize a high throughput.

[0011] To achieve the above objects, in a mobile communication system ofthe present invention, radio channels include a plurality of trafficchannels used for transmitting upward data packets directed from mobileterminals to a base station and for transmitting downward data packetsdirected from the base station to the mobile terminals, a reservationchannel used for transmitting reservation packets each indicative of atraffic channel assignment request from a mobile terminal to the basestation, and a reply channel used for transmitting reply packets eachindicative of a traffic channel through which data is transmitted andreceived from the base station to a mobile terminal, wherein thereservation, reply and traffic channels are applied with spread-spectrumin accordance with a CDMA scheme. The mobile communication system ischaracterized in that a mobile terminal having a request for datatransmission transmits a reservation packet onto the reservation channelat arbitrary timing, the base station specifies a traffic channel and atime slot to be used by the requesting mobile terminal by a reply packettransmitted through the reply channel, and each mobile terminaltransmits and receives a data packet in the time slot on the trafficchannel, both specified by the reply packet.

[0012] Describing in greater detail, each of the reservation, reply andtraffic channels is assigned a unique spreading code, for example,pseudonoise (PN). Particularly, the reservation channel is assigned aspreading code shorter than those assigned to other reply and trafficchannels. The base station relies on a matched filter to identify aplurality of reservation packet signals having time-overlapped portions,transmitted from a plurality of mobile terminals, and to perform areceiving process on bit signals corresponding to each packet.

[0013] According to a preferred embodiment of the present invention, thebase station, upon receiving a reservation packet from a mobileterminal, assigns a time slot on a traffic channel in accordance with aschedule control, and notifies each mobile terminal of the assignmentresult through a reply packet.

[0014] Also, for regulating a total number of simultaneouslycommunicated packets, the base station periodically transmits a busytone signal indicative of a traffic situation, such that each mobileterminal having a request for data transmission performs a reservationpacket transmission control in accordance with the busy tone signal.Alternatively, the radio channels may be provided with a plurality ofreply channels so as to specify a reply channel for each mobile terminalto receive the busy tone signal therethrough.

[0015] According to the present invention, time slots are defined in thetraffic channels such that each mobile terminal transmits and receivesdata in a particular time slot specified by the base station. Thereservation channel, on the other hand, is not provided with time slots,so that each mobile terminal having a request for data transmissiontransmits a reservation packet at arbitrary timing, thus facilitatingthe operation of transmitting the reservation packet in each mobileterminal.

[0016] Also, each mobile terminal performs a spectrum spreading ormultiplies the reservation packet by a spreading code to generate aspread-spectrum reservation packet, where the spreading code has aperiod shorter than that applied to a data packet transmitted through atraffic channel, while the base station receives reservation packetsusing a matched filter.

[0017] In this case, even if two or more spread-spectrum controlpackets, modulated by the same spreading code, are partially overlappedon the time axis, the matched filter can identify received packets,provided that there is a timing deviation over one chip or more on thespreading code between the respective packets. Therefore, even if aplurality of mobile terminals generate reservation packets individuallyat arbitrary timing, a reception disabled condition caused by collisionof these packets will occur with an extremely low possibility.

[0018] The foregoing and other objects, advantages, manner of operationand novel features of the present invention will be understood from thefollowing detailed description when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 illustrates an exemplary configuration of a mobilecommunication network to which the present invention is applied;

[0020]FIG. 2A is a diagram for explaining a protocol for a call set upprocess in a radio communication system according to the presentinvention;

[0021]FIG. 2B is a diagram for explaining a protocol for informationtransmission in the radio communication system according to the presentinvention;

[0022]FIG. 3 is a diagram for explaining a channel access control in aconventional radio communication system;

[0023]FIG. 4 is a diagram for explaining a channel access control in aradio communication system according to the present invention applying aCDMA scheme;

[0024]FIG. 5A illustrates a format for a reservation packet;

[0025]FIG. 5B illustrates a format for a reply packet;

[0026]FIG. 5C illustrates a format for an information transmissionpacket;

[0027]FIG. 6 is a block diagram illustrating the configuration of a basestation;

[0028]FIG. 7 is a block diagram illustrating the configuration of a CDMAtransceiver 50 in the base station;

[0029]FIG. 8A is a block diagram illustrating the configuration of amatched filter 70;

[0030]FIG. 8B is a diagram for explaining how the matched filterprocesses received reservation packets;

[0031]FIG. 9 is a block diagram illustrating the configuration of apacket separation circuit 80;

[0032]FIG. 10 is a block diagram illustrating the configuration of apacket controller 90 in the base station;

[0033]FIG. 11 is a block diagram illustrating the configuration of amobile terminal;

[0034]FIG. 12 is a block diagram illustrating the configuration of aCDMA transceiver 110 in the mobile terminal;

[0035]FIG. 13 is a block diagram illustrating the configuration of apacket controller 130 in the mobile terminal; and

[0036]FIGS. 14A and 14B are diagrams for explaining a busy tone control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037]FIG. 1 illustrates an exemplary configuration of a mobilecommunication network to which the present invention is applied.

[0038] The illustrated mobile communication network comprises a publicnetwork 1 accommodating stationary terminals such as a telephone 3 orthe like; and a mobile communication network 2 connected to the publicnetwork 1 and accommodating a plurality of base stations 4 (4 a, 4 b, .. . ), wherein each base station 4 communicates with mobile terminals(radio terminals) 5 (5 a, 5 b, . . . ) located in its service area(cell) through radio channels 6. On the radio channel, a CDMA packettransmission is applied because of its suitability to communications ofmulti-media information in which data, sound and image signals aremixed.

[0039]FIG. 2A shows a protocol for a call set up process in the radiocommunication system according to the present invention.

[0040] The call set up process includes two different sequences ofoperations: one is a sequence of operations for initially allocatinglocal ID's (local addresses) to mobile terminals in a service area, andthe other is a sequence of operations for allocating a link number toeach mobile terminal for communicating with another destinationterminal. The local ID is an address number having a reduced length thanthat of a unique address previously assigned to each mobile terminal.The use of this local ID results in reducing the length of a packet. Thelink number also has a similar effect to the local ID.

[0041] A procedure of the call set up process is common to theabove-mentioned sequences of operations for allocating the local ID'sand for allocating the link numbers. Specifically, the procedurecomprises the steps of transmitting a control packet (reservationpacket) 10 a for call set up from a terminal to a base station through areservation channel 7; transmitting a control packet (reply packet) 11 afrom the base station to the terminal through a reply channel 8; andtransmitting a call set up data packet 12 a from the base station to theterminal through a traffic channel.

[0042] Address information indicative of a source is set in the controlpacket 10 a. Also, the address of a terminal required to receive thedata packet 12 a and a time slot on the traffic channel 9 in which thedata packet 12 a is to be received, are specified by the control packet11 a, such that the terminal specified by this control packet 11 areceives the call set up data packet 12 a including locationregistration information (local ID number) or link information (linknumber) transmitted by the base station in the specified time slot onthe traffic channel 9.

[0043] It should be noted that if the control packet 11 a has asufficient length, the location registration information or the linkinformation may be transmitted through the control packet 11 a, insteadof utilizing the call set up data packet 12 a.

[0044] The reservation channel 7, reply channel 8, and traffic channels9 are distinguished by PN codes which are applied to spread-spectrum. Aplurality of traffic channels 9 can be formed by providing a pluralityof PN codes for transmitting data packets.

[0045] The base station is provided, for example, with a managementtable for indicating a slot using situation on each traffic channel suchthat the base station schedules a slot for transmitting the data packet12 a so as to minimize a waiting time of the terminal by referring tothis management table.

[0046]FIG. 2B shows a protocol for transmitting user information(hereinafter simply called the “data”).

[0047] A terminal (transmitting terminal) having a request for datatransmission utilizes a PN code for the reservation channel 7 totransmit a control packet (reservation packet) 10 b for requesting theassignment of a slot in which a data packet is to be transmitted. Thebase station, in response to this request, utilizes a PN code for theresponse channel 8 to transmit a control packet (reply packet) 8 b tothe request transmitting terminal, thereby specifying a traffic channel9 i and a time slot to be used by the request transmitting terminal. Therequest transmitting terminal, upon receiving the reply packet 11 b,sends the data packet 12 b at the timing of a specified time slot on thetraffic channel 9 i.

[0048] The data packet 12 b is once received by the base station. Thebase station confirms a destination address of the data packet, andutilizes the PN code for the reply channel 8 to transmit a controlpacket 13 for specifying a destination terminal (receiving terminal) aswell as a traffic channel 9 j and a time slot with which the receivingterminal is to receive the data packet 12 b, when the receiving terminalis a mobile terminal located in the service area of the base station.Then, the base station sends the received data packet 12 b from therequest transmitting terminal as a data packet 14 in the specified timeslot. The receiving terminal receives the data packet 14 transferredfrom the base station in the specified time slot on the traffic channel9 j specified by the control packet 13.

[0049] According to the information transmission protocol describedabove, while a data transfer in the upward direction from a transmittingterminal to a base station requires a reservation packet, a datatransfer in the downward direction from the base station to a receivingterminal does not require the reservation packet.

[0050] The base station provides each mobile terminal with referencetiming in data packet transmission/reception operations using a pilotsignal transmitted through a pilot channel in parallel with thetransmission of the data packet 14. Since each mobile terminal canreceive the data packet 14 and the pilot signal transmitted from thebase station with the same delay time, the mobile terminal can readilyaccomplish synchronization acquisition, when receiving the data packet14, by determining the timing of a receiving time slot based on thepilot signal.

[0051]FIG. 3 shows a reservation based access control in a conventionalFDMA radio communication system.

[0052] As described above in connection with FIG. 2A, the reservationbased access control is a control method in which a reservation packetis sent prior to the transmission of a data packet, and the data packetis transmitted after the reservation is established. For this control,the reservation channel 7 and the reply channel 8 are provided inaddition to the traffic channels 9. The channels may be divided inaccordance with the time division multiple access (refer to the priorart 2) other than the frequency division multiple access (refer to theprior art 1) shown in FIG. 3.

[0053] In FIG. 3, the abscissa represents the time axis 21. When a radioterminal transmits a reservation packet to a base station through thereservation channel 7, the base station schedules time slots on thetraffic channels, and transmits a reply packet indicative of areservation result to the radio terminal through the reply channel 8.

[0054] In the conventional reservation based access control, if aplurality of radio terminals transmit reservation packets onto thereservation channel 7 at a time, the reservation packets may collidewith each other and collapse, as indicated by 22 a, 22 b in FIG. 3, withthe result that the base station cannot receive the reservation packets.Each radio terminal determines that its reservation packet would havecollided with any other reservation packet on the reservation channel ifa reply packet destined thereto has not been returned in a predeterminedtime period after the radio terminal had sent the reservation packet. Inthis event, the radio terminal again transmits the reservation packet(indicated by 23 a, 23 b). Thus, the throughput in a radio communicationsystem employing the conventional reservation based access control islimited depending on the collision of reservation packets as describedabove.

[0055]FIG. 4 shows an access control in a reservation based CDMA radiocommunication system according to the present invention.

[0056] The present invention applies CDMA packet transmission to areservation channel to allow a plurality of radio terminals to transmitreservation packets individually at arbitrary timing.

[0057] In a reservation channel 7 illustrated in FIG. 4, the ordinaterepresents transmitting terminals 25. FIG. 4 represents a situation inwhich the transmitting terminals 25 have transmitted reservation packetspartially overlapped on the time axis 21.

[0058] In the CDMA scheme, the spread-spectrum is applied by replacingeach symbol (bit “1” and “0”) in transmitted data with a spreading code(orthogonal code or PN code) composed of a plurality of chips havingunique patterns. For example, in a direct sequence spread-spectrum, aplurality of transmitting terminals modulate transmission data using thesame PN (pseudonoise) sequence, and transmit the spread-spectrum data atthe same carrier frequency. In this event, if there is a time deviationof one chip or more in transmission timing between respective symbols indata, the receiving side can individually identify each of transmitteddata.

[0059] If a plurality of reservation packets are transmitted atcompletely the same timing, the packets will collide, wherebydestinations will fail to receive the reservation packets. However,generally, such transmission of a plurality of reservation packets atcompletely the same transmission timing is rather a rare case. In thespread-spectrum, even if two packets are time-overlapped, the collisionis avoided when these packets are deviated in timing by a time equal toor longer than one chip, as indicated by 26 a, 26 b in FIG. 4, thuseliminating the need to retransmit the reservation packets. It will beappreciated that the reservation based control scheme according to thepresent invention significantly improves the throughput compared withthe conventional reservation based communication system.

[0060] In the present invention, each radio terminal having a requestfor data transmission transmits a reservation packet at arbitrary timingon the reservation channel, and sends a data packet in a time slot on atraffic channel, both specified by a reply packet received through thereply channel.

[0061] The data packet is transmitted in units of time slot inprinciple. When transmission data is so long that a plurality of timeslots are required for the transmission, the data is divided into aplurality of data packets, and a time slot is reserved for each datapacket. However, for reducing overhead due to the reservation process, aplurality of time slots may be reserved by a single reservation packetsuch that a base station, in response to the reservation packet, assignsa plurality of continuous or intermittent time slots to a transmittingterminal by a single reply packet or a plurality of reply packetsgenerated for respective time slots.

[0062] While the present invention allows the mobile terminals totransmit reservation packets at arbitrary timing, the mobile terminalsmust transmit and receive a reply packet and a data packet insynchronism with a time slot having a previously defined constantlength.

[0063] As illustrated in FIG. 4, the reply channel 8 and the respectivetraffic channels 9 are divided into time slots respectively having afixed length, and a pilot signal is used to match the timing, thusfacilitating fast synchronization of spreading codes between each radioterminal and a base station. More specifically, the base station spreadsthe pilot signal (reference signal) with a spreading code (PN sequence)having a suitable period, and continuously transmits the spread-spectrumpilot signal on a common channel (pilot channel). Each radio terminalgenerates a synchronization signal based on the pilot channel despreadfrom the spread-spectrum pilot signal with a PN sequence unique to thepilot channel, and sets a time slot in synchronism with the base stationon the reply channel and on each traffic channel.

[0064] It should be noted that since the pilot signal is intended forthe synchronization of the spreading codes, the pilot signal may includeany contents. Thus, for transmitting the pilot signal, the replychannel, for example, may be utilized instead of using the dedicatedpilot channel.

[0065] FIGS. 5A-5C illustrate formats for the packets used in the mobilecommunication system according to the present invention.

[0066] The reservation packet, as illustrated in FIG. 5A, is composed ofa preamble 31 a for synchronization acquisition; a type of reservation432 b indicative of the type of the packet (identification code foridentifying a location registration packet, a link securing packet, or atraffic channel reserving packet); a source address 33 (using a local IDif the location has been registered); a destination address 34 (using alink number if a link has been secured); a number 35 of reservationdesired transmission packets (time slots); and a CRC (Cyclic RedundancyCheck) code 36 a serving as an error detection code, arranged in thisorder from the beginning. The number 35 of transmission packets is notrequired in the call set up process for location registration or linksecuring.

[0067] The reply packet, as illustrated in FIG. 5B, is composed of asource address 34; a type of reply 32 b indicative of the type of thepacket (for identifying a location registration packet, a link securingpacket, an upward direction information transmitting packet or adownward direction information transmitting packet); a PN type 37indicative of a spreading code of a traffic channel to be sued; timinginformation 38 indicative of assigned transmission timing (time slot);and a CRC code 36 b, arranged in this order from the beginning.

[0068] It should be noted that in the present invention, the replypacket does not require a preamble. This is because each radio terminalcan acquire each reply packet by receiving the pilot signal andestablishing the synchronization of each time slot on the reply channelbased on the pilot signal, as described above.

[0069] The data packet for transmitting information, as illustrated inFIG. 5C, is composed of a preamble 31 b; a type of packet (foridentifying a location registration packet, a link securing packet, anupward information transmitting packet, or a downward informationtransmitting packet) 32 c; a source address 33 (using a local ID if thelocation has been registered); a destination address 34 (using a linknumber if a link has been secured); data 39 (a PN code for theinformation transmitting channel or the reply channel, transmission orreception timing, and transmission information); and a CRC code 36 c,arranged in this order from the beginning.

[0070] Since the reply channel and the traffic channel for transmittinginformation are respectively divided into packets, it is desirable thatthe sizes of respective packets be unified to a fixed length even if thetypes of packets are different. For this purpose, dummy bits may beinserted in a front portion of each packet so as to adjust the beginningposition of respective fields subsequent thereto. In the downward datapacket, the preamble 31 b may be omitted as is the case of the replypacket.

[0071]FIG. 6 illustrates a schematic configuration of the base station4.

[0072] The base station 4 comprises an antenna 41; a CDMA transceiver50; a packet controller 90; a BSC interface 42 connected to a controller(BSC 43) intervening between the base station 4 and the mobilecommunication network 2.

[0073]FIG. 7 illustrates in detail the configuration of the CDMAtransceiver 50 in the base station. The CDMA transceiver 50 comprisesreceiving radio module 52 and a transmitting radio module 53 formodulating and demodulating a baseband signal as well as fortransmitting and receiving signals at radio frequencies.

[0074] Referring specifically to FIG. 7, a control packet (reply packet)signal transmitted from a base station to a radio terminal is inputtedto an encoder 58 a through a reply channel signal line 45 a, and issubjected to encoding for error correction using, for example, aconvolutional code or the like. The encoded reply packet signal ismultiplied by an orthogonal code for the reply channel outputted from anorthogonal code generator 59 in a multiplier 56 a to generate aspread-spectrum reply packet signal which is then inputted to an adder60.

[0075] Similarly to the reply packet signal, data packet signalsoutputted to a plurality of signal lines 45 b respectively correspondingto traffic channels are encoded in the encoder 58 b, and multiplied byorthogonal codes corresponding to respective traffic channels in amultiplier 56 b to generate spread-spectrum data packet signals whichare then supplied to the adder 60. A pilot signal outputted to a signalline 45 c is likewise encoded in an encoder 58 c, multiplied by anorthogonal code unique to the pilot channel in a multiplier 56 c togenerate a spread-spectrum pilot signal which is then supplied to theadder 60.

[0076] The output of the adder 60 is multiplied by a PN code (long code)unique to each base station outputted from a PN generator 57 a in amultiplier 56 to generate a spread-spectrum signal which is subsequentlysupplied to the transmitting radio module 53.

[0077] On the other hand, a received signal processed by the receivingradio module 52 is inputted to a matched filter 70 a for the reservationchannel and to a plurality of matched filters 70 b-70 b′ respectivelycorresponding to traffic channels.

[0078] The matched filter 70 a despreads the received signal with a PNcode unique to the reservation channel. The despread signal is separatedinto a plurality of bit data trains 89 each for a correspondingreservation packet in a packet separation circuit 80. In this case, asdescribed later with reference to FIGS. 8 and 9, if the period of a PNsequence applied to the despreading process is selected to be equal tothe number of taps of the matched filter, the outputs of the matchedfilter can be used as despread results without further processing, thusrealizing fast synchronization. Each bit data train for a correspondingreservation packet, separated from other bit data trains in the packetseparation circuit 80, is subjected to a decoding process accompanied byerror correction, for example, such as Viterbi decoding or the like in adecoder 55 a, and subsequently supplied to the packet controller 90.

[0079] The matched filters 70 b-70 b′ are provided for acquiring theinitial synchronization of PN sequences of received signal son therespective traffic channels. Once the synchronization is acquired, eachof the PN generators 57 b, 57 b′ generates a PN sequence for eachchannel in synchronism with the acquired PN sequence. The receivedsignal is multiplied by PN sequences corresponding to respectivechannels generated by the PN generators 57, 57 b in multipliers 56, 56′to be despread. The despread signals are accumulated for every onesymbol length in accumulators 54, 54′. The accumulated results aredecoded by decoders 55, 55′ and subsequently supplied to the packetcontroller 90 as data packet signals for the respective trafficchannels.

[0080]FIG. 8A illustrates the principle of the matched filter 70 a. Thematched filter 70 is composed of a plurality of cascaded delay elements71 each having a delay time T equal to a chip width of a PN sequence; aplurality of taps arranged on the input side of the delay element at thefirst stage and on the output side of the respective delay elements; anda plurality of coefficient multipliers 72, one in each tap. The matchedfilter 70 a is configured such that received signals inputted at everychip time propagate from one tap to the next in the delay time T.

[0081] In the matched filter 70 a for the reservation channel, the delaytime of each delay element 71 is equal to the chip width of a PNsequence for the reservation channel, and the number of taps is equal tothe number of chips included in one period of the PN sequence, such thata one-period portion of the PN sequence simultaneously appears at theplurality of taps at the time the top chip of an inputted signal reachesthe rightmost tap. Therefore, respective chip values (“1” or “−1”) ofthe PN sequence a1-an for the reservation channel are previously set inthe respective coefficient multipliers 72 as coefficients, and a totalsum of the results of multiplications of respective tap outputs by therespective coefficients is calculated by an accumulator 73. If theaccumulation result is outputted as a correlation value between thereceived signal and the PN sequence for the reservation channel, thesynchronization is acquired at the time the correlation value changingfor every chip time presents a peak value. Also, the output value of theaccumulator 73 at this time indicates a demodulated value generated bydespeading the received signal.

[0082] In the present invention, the number of taps of the matchedfilter 70 a is made equal to a spreading code length so that the output79 a of the matched filter 70 a contains information (symbol code) of aone-bit portion of the reservation packet. Also, a short code type PNsequence having a less number of chips is applied as a spreading codefor the reservation channel to reduce the number of taps required to thematched filter, thus facilitating the synchronization acquisition.

[0083]FIG. 8B illustrates an output signal of the matched filter 70 awhich is generated when two reservation packets A, B are partiallyoverlapped on the time axis.

[0084] The output signal 79 a of the matched filter 70 a includes aplurality of positive peak values (indicative of a code bit “1”) and aplurality of negative peak values (indicative of a code bit “0”)generated by the accumulator 70 a. Peak values equal to or more than apredetermined threshold are detected from the output of the matchedfilter 70 a and grouped into groups of signals appearing at a timeinterval matching with the PN sequence period from the respective startpoints at which the first peak values are detected (synchronizationacquisition time), thereby making it possible to identify a bit datatrain 78 belonging to the reservation packet A and a bit data train 76belonging to the reservation packet B.

[0085] In the illustrated example, the peak value 76-1 appearing firstis defined as the start point, and signal values(“1” or “−1”) 76-2,76-3, 76-4, . . . subsequently appearing at a time interval equal to thePN period 75 are extracted from the output of the matched filter 70 a toreproduce the bit data train 76 constituting the reservation packet A.Also, a peak value 77-1 appearing asynchronously with the bit data train76 is defined as the start point, and signal values (“1” or “−1”) 77-2,77-3, 77-4, . . . are extracted at a time interval equal to the PNperiod 75 are extracted from the output of the matched filter 70 a toreproduce a bit data train 77 which constitutes the reservation packetB. By applying a similar principle, even if three or more reservationpackets are transmitted in a time-overlapped condition, bit signals foreach packet can be identified as long as a phase deviation over one chipor more exists between the respective packets.

[0086]FIG. 9 illustrates an exemplary configuration of the packetseparation circuit 80.

[0087] The output signal 79 a of the matched filter 70 a is inputted toan absolute value circuit (ABS) 81, the output of which is compared witha predetermined threshold outputted from a threshold circuit 82 by acomparator 83 a. When the output of the absolute value circuit 81 islarger than the threshold, the output of the comparator 82 is turned ON(“1” state) and inputted to an AND circuit 84 a. Since the AND circuit84 a is also supplied, as other input signals, with inverted signalswhich are initially OFF (“0” state), the AND circuit 84 is opened by theON output from the comparator 83 a, whereby its output signal is turnedON (“1” state). The ON output from the AND circuit 84A is inputted toAND circuits 84 b and 84 d.

[0088] The AND circuit 84 b is also supplied at the other input terminalthereof with an inverted version of an output signal from a timer 85 a.In an initial state, the output of the timer 85 a is in OFF state (“0”state), so that the output of the AND circuit 84 b is also turned ON atthe time the output of the AND circuit 84 a is turned ON. The ON outputof the AND circuit 84 b is inputted to a timing register 86 a as anenable signal, whereby the timing register 86 a is set at a valuerecorded on a counter 87 which performs a counting operation at aninterval equal to the chip period of the PN code and returns to aninitial value at an interval equal to the symbol length. The counter 87outputs a value which indicates a chip position at the timing at whichthe synchronization is acquired, as previously described with referenceto FIG. 8B.

[0089] The ON output of the AND circuit 84 b causes a timer 85 a tostart for controlling the other input terminals of the AND circuits 84 band 84 d. The timer 85 a maintains its output in ON state for a timeperiod corresponding to one reservation packet. This permits the ANDgate 85 d to remain open and the AND gate 84 b to remain close until atime set in the timer 85 a expires, thus preventing any other countedvalue from being set in the first timing register 86 a.

[0090] If the next peak value is outputted from the matched filter 70 abefore the time set in the timer 85 a expires, the ON output from theAND circuit 84 a is inputted to an enable terminal of a second timingregister 86 b through a pair of AND circuits 84 d and 84 d′ which remainopen. As a result, the output value of the counter 87 is set in thesecond register 86 b. At this time, a timer 85 b cooperating with thesecond timing register 86 b is started and performs a similar operationto that of the timer 85 a to prohibit any other value from being set inthe second timing register 86 until a one-packet period has elapsed andto open a pair of AND gates at the next stage so as to input thesubsequently generated enable signal to a third timing register 86 c.

[0091] In this embodiment, since the packet separation circuit 80 isprovided with four timing registers 86 a-86 d, the synchronizationacquisition timing is stored for four reservation packets, determined bythe order of generation, within a plurality of reservation packetsgenerated in a time-overlapped condition by repeating the foregoingoperations in a similar manner.

[0092] The value of the synchronization acquisition timing set in thetiming register 86 a is compared with an output value of the counter 87in a comparator 83 b. Every time the counted value is coincident withthe synchronization acquisition timing value set in the timing register86 a, the output of the comparator 83 b is turned ON.

[0093] The ON output of the comparator 83 b is inputted to an enableterminal of a data register 87 a through the AND circuit 84 c whichremains open while the timer 85 a is in ON state. As a result, the dataregister 87 a is supplied with the output of the matched filter 80 a atthe synchronization acquisition timing. The remaining timing registers86 b-86 d also operate in a manner similar to the foregoing to store theoutputs of the matched filter 70 a for respective reservation packets indata registers 87 b-87 d, respectively.

[0094] Since the data registers 87 a-87 d are supplied with data inaccordance with the synchronization acquisition timing of the respectivereservation packets, the contents of these data registers 87 a-87 d aretransferred to output registers 88 a-88 d, respectively, in synchronismwith a clock having a bit period generated by a clock generator 88, anddata indicative of the contents of the respective reservation packetsare transferred to the decoder 55 a illustrated in FIG. 7 from theoutput registers 88 a-88 d.

[0095]FIG. 10 illustrates an exemplary configuration of the packetcontroller 90 in the base station 4.

[0096] Received data from the reservation channel (the contents of areservation packet) is inputted to a digital signal processor (DSP) 91,and is processed by a reservation packet processing routine 92 of theDSP 91. Subsequently, an assignment of a traffic channel and a time slot(scheduling) is performed by an upward schedule control routine 93.

[0097] A traffic channel (PN type) and a time slot (timing information)determined by the upward schedule control routine 93 is transferred to areply packet constructing unit 97 together with a source address of areservation packet to which a reply packet is destined. The reply packetconstructing unit 97 generates a reply packet including the aboveinformation and transmits it to the reply channel signal line 45 a. Inthis way, the operation for transmitting an upward data packet from eachmobile terminal can be controlled in accordance with the scheduling ofthe base station.

[0098] Received data from respective traffic channels are inputted toreception processing units 96 b, 96 b′ arranged in correspondence to therespective traffic channels through signal lines 44 b, 44 b′, andtransferred to the BSC interface 42 through signal lines 46 as receiveddata packets.

[0099] On the other hand, a downward data packets outputted from the BSCinterface 42 to signal lines 47, after temporarily stored intransmission buffers 99, 99′, are transmitted under the control of aschedule executed by a downward schedule control routine 95 of the DSP91. More specifically, in accordance with a downward schedule, a replypacket constructed by the reply packet constructing unit 97 is firstsent from the reply channel, and subsequently data packets generated bythe traffic packet constructing units 98 a, 98 a′ are sent inpredetermined time slots on traffic channels determined by the downwardschedule.

[0100] In this embodiment, for restraining mobile terminals from issuingreservation packets when the traffic channels remain busy, a busy tonevalue calculation routine 94 of the DSP 91 generates busy toneinformation in accordance with the number of reservation packetsreceived through the reservation channel and traffic channel utilizationstate information known to the upward schedule control routine 93, andnotifies the busy tone information to the respective mobile terminalsthrough the reply channel 45 a.

[0101]FIG. 11 illustrates the configuration of the radio terminal 5.

[0102] The radio terminal 5 is composed of an antenna 100; a CDMAtransceiver 110 connected to the antenna 100; a packet controller 130connected to the CDMA transceiver 110; and a data processing unitconnected to the packet controller 130.

[0103] The data processing unit comprises a microprocessor (MPU) 101; amemory 102 for storing data and programs; and a plurality ofinput/output devices connected to an internal bus through an I/Ointerface 103. The input/output devices may comprise, for example, acamera 104 a, a speaker 104 b, a display 104 c, a keyboard 104, and soon.

[0104]FIG. 12 illustrates in detail the configuration of the CDMAtransceiver 110 in the radio terminal.

[0105] The CDMA transceiver 110 comprises a receiving radio module 112and a transmitting radio module 113. These modules are responsible formodulation or demodulation of a baseband signal and a receiving processor a transmitting process at radio frequencies.

[0106] In a transmitter circuit, a reservation packet signal outputtedto a reservation channel signal line 106 a is encoded for errorcorrection in an encoder 120 a, and then multiplied by a unique PNsequence (short code) generated from a PN generator 121 a in amultiplier 114 a to generate a spread-spectrum reservation packet signalwhich is sent to the transmitting radio module 113.

[0107] On the other hand, a data packet outputted to a traffic channelsignal line 106 b is encoded for error correction in an encoder 120 b,and multiplied by a PN sequence (long code) generated by a PN generator121 b in a multiplier 114 b to generate a spread-spectrum data packetwhich is sent to the transmitting radio module 113. The spread-spectrumfor the data packet is performed using a PN sequence specified by a basestation, which is identified by a control signal outputted onto a signalline 106 c by a packet controller 130 and in synchronism with referencetiming 105 c provided from a PN generator 119 in a receiver circuit.

[0108] In the receiver circuit, a received signal outputted from thereceiving radio module 112 is inputted to a multiplier 114 c whichmultiplies the received signal by a PN code unique to the base stationgenerated by the PN generator 119 to despread the received signal. Theoutput of the multiplier 114 c is parallelly inputted to multipliers 114d, 114 e and 114 f respectively for the reply channel, traffic channelsand pilot channel, and multiplied by orthogonal codes unique to therespective channels generated by an orthogonal code generator 117.

[0109] On a reply channel line 105 a and a traffic channel line 105 b,output signals from the multipliers 114 d, 114 e are inputted toaccumulators 115 d, 115 e, respectively, to produce accumulated valuesfor each symbol length for despreading the output signals from themultipliers 114 d, 114 e. Output signals of the respective accumulators115 d, 115 e are inputted to decoders 116 d, 116 e, respectively, forerror correction, and then transferred to the packet controller 130through signal lines 105 d, 105 e, respectively.

[0110] On a pilot channel line 122, a pilot signal outputted from anaccumulator 115 f is inputted to a DLL (Delay Locked Loop) circuit 118for tracking of synchronization. The PN generator 119 is forced togenerate a PN sequence in synchronism with the output of the DLL circuit118. It should be noted that the decoders 116 d, 116 e on the replychannel line 105 a and the traffic channel line 105 b are operated insynchronism with the pilot signal outputted from the accumulator 115 f.

[0111]FIG. 13 illustrates an exemplary configuration of the packetcontroller 130 in the radio terminal.

[0112] Received data through the reply channel appearing on the signalline 105 a is inputted to a DSP 131 and precessed by a monitoringroutine 132. The contents of the reply packet is supplied to an upwardschedule control routine 134 and to a downward schedule control routine135, while a busy tone signal received through the reply channel issupplied to a busy tone calculation routine 133.

[0113] Received data through a traffic channel appearing on the signalline 105 b is received by a reception processing circuit 136 which iscontrolled by a control signal from the downward schedule controlroutine 135 and a reference timing signal 105 c, and received data in aparticular time slot specified by a base station through a reply packetis outputted onto a signal line 107 as receiving information.

[0114] On the other hand, transmission data from the radio terminal,after temporarily stored in a transmission buffer 138, is fetched by atraffic packet constructing unit 139 in accordance with an instructionfrom the upward schedule control routine 134, and is sent onto thetraffic channel signal line 106 b as a data packet.

[0115] When a reply packet is received from a base station, the upwardschedule control routine 134 generates a signal 106 for specifying atraffic channel (PN sequence) to which a traffic packet is to be sent,and issues a data packet sending instruction to the traffic packetconstructing unit 139 at timing of a time slot specified by the basestation. The traffic packet constructing unit 139, upon receiving thedata packet sending instruction from the control routine 134, readstransmission data from the transmission buffer 138, and sends the datapacket illustrated in FIG. 5C onto the traffic channel signal line 106 bat predetermined output timing determined based on the reference timingsignal 105 c.

[0116] The busy tone value calculation routine 133 calculates a busytone value indicative of a traffic situation from a busy tone signalreceived through the reply channel, and notifies the busy tone value tothe upward schedule control routine 134.

[0117] The upward schedule control routine 134 controls the generationof reservation packets in accordance with the traffic situation. Forexample, if the busy tone signal does not indicate to restrain datatransmission with transmission data being accumulated in thetransmission buffer, the reservation packet constructing unit 137 isstarted at arbitrary timing to transmit a reservation packet to thereservation channel signal line 106 a. Conversely, if the busy tonesignal indicates to restrain data transmission, the transmission ofreservation packets is restrained until the traffic situation improves.

[0118] As described above, in this embodiment, the CDMA scheme isapplied to the reservation channel to reduce the possibility ofretransmission of reservation packets due to collision of thereservation packets even if respective mobile terminals transmit thereservation packets at arbitrary timing. Moreover, the busy tone controlis added to restrain the transmission of new packets from mobileterminals when the traffic channels or the reservation channel is in anoverload condition.

[0119] The CDMA has a problem that when a plurality of packets aregenerated in a time-overlapped condition, the packet signals mutuallyaffect as noise, so that if a large number of packets are simultaneouslygenerated, the receiver side cannot identify them because all packetsignals are buried in noise. As described above, in the mobilecommunication system of the present invention comprising a reservationchannel, a reply channel and a plurality of traffic channels, the totalnumber of reply packets and data packets can be controlled by thescheduling function of the base station, whereas the base station cannotdirectly control reservation packets since they are issued autonomouslyfrom respective mobile terminals.

[0120] As described above, a method which allows each radio terminal toautonomously control the transmission of a reservation packet withreference to the busy tone signal from the base station is effective inavoiding concentrated reservation packets to smoothly control thetransmission in each terminal.

[0121] While the busy tone signal may be transmitted through a channeldedicated thereto, empty time zones appearing periodically on the replychannel may also be utilized.

[0122] The reply channel, as shown in FIG. 2, is divided into time slotseach having a length corresponding to the length of a data packet on thetraffic channel based on the pilot signal. Since the reply packetincludes a smaller amount of information, its length can be made shorterthan the data packet. For example, assuming that the time slot length(data packet length) is 512 bits and the reply packet length is 42 bits,12 reply packets can be transmitted through the reply channel during onetime slot period on the traffic channel, with a 8-bit empty time zoneremaining at the end of the time slot. It is therefore possible toutilize the available empty time zone in the time slot to periodicallytransmit the busy tone signal through the reply channel.

[0123] Next, a reservation packet restraining method using the busy tonesignal transmitted in an empty time zone on the reply channel will bedescribed with reference to FIGS. 14A, 14B.

[0124] In FIG. 14B, “t−1”, “t” and “t+1” designate time slot numbers onthe reply channel, and a pulse waveform represents the busy tone signal143. The busy tone signal 143 is periodically transmitted utilizing anempty time zone left in each time slot on the reply channel.

[0125]FIG. 14A shows a relationship between a total amount of packetssent out by radio terminals in each time slot and a number T of allowedpackets which can be transmitted in a time-overlapped condition. An area148 indicates an amount of reservation packets sent in the time slot“t−1” and an area 149 indicates an amount of data packets sent in thetime slot “t−1”.

[0126] In the following, the busy tone signal generated by the basestation in the time slot “t−1” will be described, assuming that a numberof transmitted data packets during the time slot “t” is I(t), a numberof transmitted reservation packets is R(t), a number of transmissionrequested reservation packets is R(t)′, and a transmission probabilityof reservation packets is P(t). Further, R(t)′ and R(t) are defined tobe numbers of reservation packets when the length of the reservationpacket is normalized by the length of the data packet.

[0127] First, assume the following equation (1): $\begin{matrix}{{R(t)}^{\prime} = \frac{R\left( {t - 1} \right)}{P\left( {t - 1} \right)}} & (1)\end{matrix}$

[0128] Assuming that the number R(t)′ of transmission requestedreservation packets possessed by all radio terminals in the service areaof a base station in the time slot “t” is equal to a number R(t−1)′ oftransmission requested reservation packets in the previous time slot“t−1”, the equation (1) is derived by substituting a number R(t−1) ofreservation packets actually received by the base station as the valueof R(t−1)′. To the base station, the number I(t) of data packets in thetime slot “t” is known from previously received reservation packets andthe result of scheduling the traffic channels for received data packetsfrom other base stations.

[0129] Thus, the value of R(t)′ is estimated from the equation (1), andwhen a total amount of the number R(t)′ of transmission requestedreservation packets and the number I(t) of data packets in the time slot“t” exceeds a tolerable value T as shown by the following equation (2),the transmission of reservation packets is restrained by the busy tonesignal:

I(t)+R(t)′≧T  (2)

[0130] In this event, the transmission of reservation packets iscontrolled by the busy tone signal such that the transmissionprobability P(t) of reservation packets from radio terminals in theservice area is restrained by a traffic amount on the traffic channels,as shown by the following equation (3), thereby making the sum of thenumber of reservation packets and the number of data packetssubstantially equal to the tolerable value T. Since the number ofreservation packets actually transmitted from radio terminals isdetermined from the probability, it is desirable that the tolerablevalue T be set at a slightly lower level in order to allow for a certainmargin. $\begin{matrix}{{P(t)} = \frac{\left\{ {T - {I(t)}} \right\}}{{R(t)}^{\prime}}} & (3)\end{matrix}$

[0131] On the other hand, if a total amount of packets estimated in thetime slot “t” is in a relationship expressed by the following equation(4), the transmission of reservation packets is controlled by the busytone signal such that the transmission probability P(t) follows theequation (5), thus allowing all radio terminals to freely transmitreservation packets.

I(t)+R(t)′<T  (4)

P(t)=1.0  (5)

[0132] The base station may notify respective radio terminals ofinformation indicative of the transmission probability expressed by theequation (3) or (4) as the busy tone signal 143 in the time slot “t−1”.

[0133] As will be apparent from the foregoing description, the presentinvention applies CDMA to a reservation based packet access control typemobile communication system to reduce the possibility of retransmissionof reservation packets due to their collision, even if each mobileterminal is allowed to transmit a reservation packet at its arbitrarytiming, to improve the throughput.

[0134] According to the present invention, for example, a shortspreading code is applied to a reservation packet, and thesynchronization is acquired on the base station side using a matchedfilter, so that even if a plurality of mobile terminals transmitreservation packets asynchronously to each other, the base station canidentify the respective reservation packets at a high speed. Also, areduced local address (own address) shorter than an original addressnumber or a link number (destination address) is used for terminaladdress information set to each packet, so that the transmissionefficiency can be improved. Further, when each terminal is allowed tocontrol the transmission of reservation packets in accordance with abusy tone signal from a base station, it is possible to avoid anexcessive amount of reservation packets simultaneously communicated on achannel, thus ensuring a favorable communication environment.

[0135] It is to be understood that the above-described embodiments aremerely illustrative of the principles of the invention and that mayvariations may be devised by those skilled in the art without departingfrom the spirit and scope of the invention. It is therefore intendedthat such variations be included within the scope of the claims.

What is claimed is:
 1. A communication method in a code divisionmultiple access (CDMA) mobile communication system wherein radiochannels between a base station and a plurality of mobile stationsinclude a plurality of traffic channels used for transmitting upwarddata packets directed from the mobile terminals to the base station andfor transmitting downward data packets directed from the base station tothe mobile terminals, a reservation channel used for transmittingreservation packets each indicative of a traffic channel assignmentrequest from a mobile terminal to the base station, and a reply channelused for transmitting reply packets each indicative of a traffic channelthrough which data is to be transmitted and received from the basestation to a mobile terminal, and said reservation, reply and trafficchannels are assigned unique spreading codes, respectively, said methodcomprising the steps of: transmitting a reservation packet from a mobileterminal having a request for data transmission onto said reservationchannel at arbitrary timing; transmitting a reply packet onto said replychannel from the base station to specify a traffic channel and a timeslot to be used to each mobile terminal which has transmitted saidreservation packet; and transmitting a data packet from each mobileterminal which has transmitted said reservation packet in the specifiedtime slot on the traffic channel specified by said reply packet.
 2. Acommunication method according to claim 1, wherein: said each mobileterminal having a request for data transmission applies a spreading codeunique to said reservation channel having a period shorter than aspreading code applied to the spreading of a data packet to betransmitted on said traffic channel in order to apply spread-spectrum tosaid each reservation packet; and said base station processes a receivedsignal using a matched filter in which said spreading code unique tosaid reservation channel is set, said base station utilizing theperiodicity of said spreading code to separate outputs of said matchedfilter into a plurality of signal trains corresponding to respectivereservation packets.
 3. A communication method according to claim 1,further comprising the steps of: giving a local address shorter thanunique address information assigned to each mobile terminal from saidbase station to said each mobile terminal; wherein said base stationsets a local address in a destination address field of said reply packetand transmits said reply packet; and receiving the packet including alocal address given to each mobile terminal in the destination addressfield by said each mobile terminal.
 4. A communication method accordingto claim 1, further comprising the steps of: giving a link numbershorter than address information unique to each mobile terminal fromsaid base station to said each mobile terminal; wherein said each mobileterminal transmits a data packet including said link number as adestination address onto a traffic channel.
 5. A communication methodaccording to claim 1, further comprising the steps of: periodicallytransmitting from said base station busy tone information indicative ofa traffic situation in its service area through said reply controlchannel or through a channel dedicated to the busy tone information;wherein each mobile terminal having a request for data transmissioncontrols the transmission of a reservation packet based on said busytone information.
 6. A communication method according to claim 5,wherein: said base station estimates, based on a number of reservationpackets received during a previous constant period, a number ofreservation packets to be generated in the next constant period, andgenerates said busy tone information based on said estimated value and anumber of packets scheduled to be transmitted during the next constantperiod.
 7. A communication method according to claim 1, wherein: saideach mobile terminal requests the transmission of a plurality of datapackets through a single reservation packet.
 8. A communication methodaccording to claim 3, wherein: said base station uses a control packetfor giving a local address to a mobile terminal to specify a replychannel through which the mobile terminal is to perform receivingoperations,
 9. A communication method according to claim 1, wherein:said base station continuously transmits a pilot signal including asynchronization signal component through a pilot channel or through saidreply channel such that each mobile terminal identifies a time slot onsaid traffic channel based on said received pilot signal.
 10. Acommunication method in a radio communication system wherein a basestation and a plurality of radio terminals communicate through areservation channel, a reply channel and a plurality of traffic channelsformed in accordance with a code division multiple access (CDMA) schemein radio channels, said method comprising the steps of: transmittingreservation packets onto said reservation channel from said respectiveradio terminals requesting data transmission asynchronously with eachother; after separating a plurality of reservation packet signals havingpartially overlapped portions on a time axis, received through saidreservation channel, corresponding to reservation packets, andperforming a receiving process on the reservation packets, transmittingfrom said base station a reply packet for specifying a traffic channeland a time slot to be used to each radio terminal which is a source ofeach received reservation packet through said reply channel; andtransmitting a data packet in a specified time slot on a traffic channelspecified by said reply packet from said each radio terminal.
 11. Acommunication method according to claim 10, further comprising the stepsof: after transmitting, from said base station to a radio terminalidentified by a destination address in a data packet received from saidtraffic channel, said reply packet for specifying a traffic channel anda time slot to be used by said ratio terminal for reception through saidreply channel, transmitting said received data packet in said specifiedtime slot on said specified traffic channel; and receiving the datapacket in said specified time slot on said traffic channel specified bysaid reply packet at the radio terminal which is the destination of saidreply packet.
 12. A radio terminal apparatus for communicating with abase station through radio channels comprising: means for transmitting areservation packet processed with a spreading code unique to areservation channel to said base station asynchronously with said basestation; means for receiving a reply packet processed with a spreadingcode unique to a reply channel, transmitted from said base station atpredetermined time slot timing synchronized with said base station; andmeans for transmitting and receiving a data packet processed with aspreading code unique to a particular traffic channel at predeterminedtime slot timing on said particular traffic channel specified by saidreply packet.
 13. A base station for communicating with a plurality ofradio terminals, each having a unique address, using spread-spectrumpackets through a reservation channel, a reply channel and a pluralityof traffic channels, each of said channels corresponded to a uniquespreading code, comprising: means for receiving reservation packetsignals each for requesting a traffic channel access transmitted by theplurality of radio terminals asynchronously with each other through saidreservation channel, and separating said reservation packet signals intoindividual reservation packets for a receiving process; means fortransmitting through said reply channel a reply packet for specifying atraffic channel and a time slot to be used to a radio terminal which isa source of said reservation packet; and means for receiving a datapacket transmitted from a radio terminal or for transmitting a datapacket to a radio terminal in each time slot on a traffic channel.
 14. Amobile communication system comprising a base station and a plurality ofmobile terminals, radio channels between the base station and theplurality of mobile terminals comprising a plurality of traffic channelsused for transmitting upward data packets directed from mobile terminalsto a base station and for transmitting downward data packets directedfrom the base station to the mobile terminals, a reservation channelused for transmitting reservation packets each indicative of a trafficchannel assignment request from a mobile terminal to the base station,and a reply channel used for transmitting reply packets each indicativeof a traffic channel through which data is transmitted and received fromthe base station to a mobile terminal, each of said reservation, replyand traffic channels being assigned a unique spreading code inaccordance with a code division multiple access (CDMA) scheme, wherein:said each radio terminal comprises first means for transmitting areservation packet onto said reservation channel at arbitrary timingwhen a request for data transmission is issued, and a second means fortransmitting and receiving a data packet in a time slot specified by areply packet transmitted from the base station through said replychannel; and said base station comprises first means for separating aplurality of reservation packet signals having partially overlappedportions on a time axis, received through said reservation channel, intoreservation packets and performing a receiving process on saidreservation packets, and fourth means for transmitting a reply packetfor specifying a traffic channel and a time slot to be used, throughsaid reply channel, to each radio terminal which is a source of eachreceived reservation packet.
 15. A mobile communication system accordingto claim 14, wherein: said first means of said each mobile terminalapplies a “spreading code unique to the reservation channel” having aperiod shorter than a spreading code applied to a data packettransmitted through the traffic channel by said second means to spreadsaid each reservation packet; and said third means of said base stationprocesses a received signal with a matched filter in which saidspreading code unique to the reservation channel is set, and separatesan output of said matched filter into a plurality of signal trainscorresponding to said packets utilizing the periodicity of saidspreading code.
 16. A mobile communication system according to claim 14,wherein: said base station includes means for periodically transmittingbusy tone information indicative of a traffic situation in its servicearea through said reply control channel or through a channel dedicatedto the busy tone information; and said each mobile terminal includesmeans for controlling the transmission of a reservation packet based onsaid busy tone information.
 17. A mobile control system according toclaim 16, wherein: said base station includes means for estimating,based on a number of reservation packets received during a previousconstant time period, a number of reservation packets to be generatedduring the next constant time period; and said busy tone information isgenerated based on said estimated value and a number of packetsscheduled to be transmitted during the next constant time period.